Thermostats are used to control a variety of heating and cooling systems. A variety of thermostats have been developed for use with the various types of heating systems. For example, domestic gas or electric furnaces often include a transformer that steps 115 VAC down to 24 VAC for operating the furnace relay. Some of these furnaces provide 24 VAC between a common wire and a heating transformer power wire. Other furnaces do not provide a common wire, but provide 24 VAC between the heating load wire and the heating transformer power wire. Gas millivolt systems do not typically provide 24 VAC, but only a very small-signal voltage between the heating load wire and the heating transformer power wire. Many thermostats are designed to work with a particular type of system, even though they all generally operate by closing or opening a connection between the heating load wire and the heating transformer power wire to turn the furnace on or off, respectively.
If a user wants to replace a thermostat in a heating system, he generally needs to know what type of system he has. Hardware stores need to stock a variety of thermostats for the various applications, and the user often has to return home to determine what sort of heating system he has, and hence what sort of thermostat he needs. It is therefore desirable to provide a thermostat that can be used in a variety of heating systems.